So I don't know if I'm the only one but when I look at Galio, I see a champion with no real identity and almost nonsensical abilities. He is a guardian right? Why does he selfishly heal himself when others take damage? I mean every role you look at him he has short comings. Top lane tank he doesn't have sustained damage and just cannot compete with the current fighters. In mid his early game is strong but building him glass cannon for damage means he falls off fast. And as a support he can only assist properly when his ult is up and his shield really helps himself more than his allies. Here are my thoughts on some things that could help him as a support, which is what I assume he should be.
Passive: Runic skin
Although I like that he converts magic resist into ability power thats just not really enough anymore. I believe to help him out a bit more he should additionally convert Armor into bonus magic damage on hit. If he is going to be disadvantaged by melee range attacks at least give him some pay off for getting up close.
Q: Resolute Smite
I think this ability is fairly good as is damage wise and with the slow, but I think adding the new immobilize debuff to this could be a good addition without adding too much power. Its about time there was a supp with a not so easy to land skill shot that can slow down champs like Kalista.
Of all his skills this one makes the least sense. "A guardian is always prepared" to let you die while I get all my health back. Best guardian. I think the shield should still grant the bonus armor and magic resist to allied champions, but instead of always healing Galio let it heal whoever was shielded for a % of the damage taken at the end of the shields duration. Enemies have a chance to kill them while the shield is active but at the end it can shift the fight. It also means Galio in urf can't just heal eternally and never die.
E: Righteous Gust
I think just adding some speed isn't enough anymore. It still should but I think it should have some more oomph. I think a slight knockback to enemies hit is a good move. Galio gains a small disrupt and just that tiny bit of extra cc. Hitting enemies with his q would nullify the knockback thanks to the immobolize thus not making an ult initiate any more difficult.
R: Idol of Durand
I think his ult is great and really have nothing bad to say about it. The rest of his kit was where I took issue.
Anyway these are just my thoughts I just want riot to notice that Galio is in need of an update but doesn't require a full rework.